The present invention relates to the field of optical correlators and more particularly to joint transform correlators.
In a conventional joint transform correlator (JTC), the input signal to be identified, usually a two dimensional image of a scene, and the reference signal to be compared to the input signal, are displayed in the same plane. The output of the JTC consists of a term which is the sum of the autocorrelation of the input scene and the autocorrelation of the reference signal that is formed on the optical axis (DC terms). The other two terms are the cross-correlation functions between the input signal and the reference signal that are diffracted off-axis. These two cross-correlation terms are positioned away from the optical axis and are symmetrical thereto. They indicate the presence and the locations of identified input targets. JTC are well known in the art; see for example U.S. Pat. No. 4,695,973, issued to Francis T. S. Yu, and U.S. Pat. No. 4,832,447 issued to Bahram Javidi.
Ideally, a correlation peak would occur in the JTC output plane only at the location of each target in the input scene. However, if there are multiple targets in the input scene, it is well known that spurious auto-correlation signals can be generated among these multiple targets themselves, each correlating with one another in all permutations and combinations. One example of this is a scene with many prominent repetitions of a feature, such as a picture of a house with a picket fence. This causes false alarms and degrades the performance and reliability of the JTC.
A copending allowed patent application, entitled METHOD OF REMOVING SPURIOUS RESPONSES FROM OPTICAL JOINT TRANSFORM CORRELATORS, application Ser. No. 08/083,152, filed Jun. 25, 1993, and assigned to the same assignee as the present invention, discloses a technique for eliminating the aforesaid problem posed by the spurious correlation signals. This technique (hereinafter the Javidi invention) is also disclosed in substantial detail in "Technique for Reducing the Redundant and Self-Correlation terms in Joint Transform Correlators", Tang and Javidi, Applied Optics, 10 Apr. 1993, Vol. 32, No. 11, pages 1911-1918. The Javidi invention causes the optical path length between the reference image plane and the Fourier transform lens to differ from the optical path length between the input image plane and the Fourier transform lens. This produces three principal output correlation planes. The optical path difference is such as to focus a first off-axis true correlation peak signal in a first output plane of the JTC and a second off-axis true correlation signal in a second plane separated from the first output plane. The spurious correlation peaks which cause false alarms, will not appear in these output planes but instead in a third plane situated midway between the aforesaid first and second planes. Thus, only correlation peaks resulting from features in the reference image correlating with features in the input scene image will be in focus in the output plane. In the principle embodiment of the Javidi invention, the reference image plane is physically displaced along the optical axis relative to the input image plane. In a second embodiment, both input images lie in the same plane so that a single spatial light modulator (SLM) can be used and a rectangular block of glass is positioned in front of the reference image to create the desired optical path length difference.
It is deemed desirable to provide an alternative technique for providing a virtual displacement of the JTC reference image plane relative to the input image plane to solve the aforesaid problem discussed above in connection with the Javidi invention. A simple system is desired to produce the virtual input displacement and whereby the length of the virtual displacement can be easily varied over a substantial range to provide for considerable flexibility in the design parameters selected for the JTC and thus a substantial reduction of JTC design constraints.